Cyclooxygenases catalysis

Conversion of arachidonate to PGH via cyclooxygenase catalysis is a key regnlatory step in prostanoid biosynthesis. 

In order for the cyclooxygenase to function, a source of hydroperoxide (R—O—O—H) appears to be required. The hydroperoxide oxidizes a heme prosthetic group at the peroxidase active site of PGH synthase. This in turn leads to the oxidation of a tyrosine residue producing a tyrosine radical which is apparendy involved in the abstraction of the 13-pro-(5)-hydrogen of AA (25). The cyclooxygenase is inactivated during catalysis by the nonproductive breakdown of an active enzyme intermediate. This suicide inactivation occurs, on average, every 1400 catalytic turnovers. 

Kurumbail RG, Kiefer JR, Mamett LJ. Cyclooxygenase enzymes catalysis and inhibition. Curr Opin Struct Biol 2001 11 752-60. 

The mode of action of these compounds, which are under investigation, is via an antioxidative mechanism. Since COX enzyme catalysis involves radical intermediates, a radical scavenging moiety such as a di-fenf-butylphenol interferes with the cyclooxygenase reaction. Linkage of phenolic substructure with a thiazolone, oxazolone, thiadiazole or oxadiazole derivative produces non-... 

A useful insight into the subtle differences between the catalytic sites of the two enzymes was provided by the observations that while the cyclooxygenase activities of both were inhibited by aspirin, aspirin-acetylated Cox-2 has an increased 15-oxygenase activity, which can be inhibited by some Cox inhibitors (Mancini et al., 1994 Lecomte et al., 1994). Aspirin is known to acetylate Ser-530 of Cox-1 and Ser-516 of Cox-2. Apparently, in the acetylated Cox-2 catalytic site there is still room for arachidonate to enter in an extended conformation, which permits catalysis of 15-hydroperoxyeicosatetraenoic acid formation but not of PGG2 formation. This led to the conclusion that the catalytic site of Cox-2 must be slightly larger than that of Cox-1, a conclusion later... 

The inhibitors of COX enzymes are called nonsteroidal antiinflammatory drugs (NSAIDs) that are prescribed to relieve pain and fever. They stop prostaglandin and thromboxane production. Acetylsalicylic acid (aspirin) was used for this for many years and it was eventually discovered to acetylate a serine residue involved in the dioxygenase action of cyclooxygenases. A second class of NSAIDs, typified by ibuprofen (commonly called Advil or Motrin), inhibits catalysis by attaching irreversibly to cyclooxygenases. Aspirin and ibuprofen inhibit all prostaglandin and thromboxane synthesis. 

An unusual kinetic feature of COX is the autoinactivation (suicide inactivation) of the enzyme both the cyclooxygenase and peroxidase activities are inactivated during catalysis as the result of non-productive breakdown of active enzyme intermediates. The chemical changes in the protein that accompany this process are unknown, and although the biological relevance of this inactivation is unclear, it may constitute a cmde regulatory mechanism of cellular prostaglandin biosynthesis (Smith et al, 2000). 

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